This invention relates generally to methods for monitoring flood front movement during secondary and tertiary oil recovery and more specifically to methods for monitoring salinity and oil saturation changes and directional flood front movement of water injected into subsurface formations.
In the production of oil from subsurface locations it is well known that frequently primary production methods are ineffective in recovering substantially all the oil within a reservoir. The depletion of the reservoir energy before the depletion of the recoverable oil leaves a portion of the oil in the ground without a natural propulsive energy to move it. After the reservoir energy is about exhausted, and the production is approaching its economic limit, much of the remaining oil may be recovered by supplying a new energy source. One such outside source of energy utilized in secondary and tertiary recovery operations is the injection of water into the subsurface formations.
Water flooding depends on the ability of injected water to displace the oil remaining in the reservoir in the same manner it displaces oil in the primary production of a water-drive reservoir. Water is injected into the reservoir through a number of intake wells located at spaced intervals. As the injected water enters the reservoir, it moves toward the area of lower fluid potential and, as it moves, drives the oil left behind during the primary recovery phase. An increased oil saturation develops ahead of the moving water and finally reaches the production wells.
In performing a water flooding operation it is important to monitor the progress of the flood front to determine the lateral movement thereof. Due to formation characteristics, the flood front does not move in uniform fashion from the injection wells toward the production well. Further, subsurface formations may contain high-permeability streaks which allow injected water to break through the oil into the production well. The result of such a breakthrough is the production from the well of water while significant oil may remain in the formations.
In the prior art, various methods have been utilized to monitor the progress of the flood front in secondary and tertiary oil recovery operations. One such method as disclosed in U.S. Pat. No. 3,874,451, issued to Jones et al, detects the arrival of the flood front by monitoring the pressure change in boreholes. A requirement of Jones et al is that the boreholes used for pressure monitoring must be uncased. In a production reservoir this can require the removal of casing already present in the boreholes or the drilling of new, uncased boreholes.
U.S. Pat. No. 4,085,798, issued to Schweitzer et al discloses a method for monitoring the flood front profile during water flooding by adding a tracer element having a characteristic gamma ray emission energy to the flood fluid. The tracer element may be unlike any element nomrally found in the formation, or it may be an element similar to elements normally present in the formation. It is recognized as a serious disadvantage to be required to add tracer elements to the flood fluid prior to injection. Additionally, since the Schweitzer method is only directed to detecting elements in the injection fluid it does not provide an indication of flood front movement until the fluid flood front reaches or nearly reaches the monitor boreholes.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a method for monitoring the flood front movement through cased boreholes without alteration of the injection fluid.